Rigging: moving the leg moves other parts as well
Ah, this a common issue with automatic weights. It is easily fixable though. Select the armor piece that is moving when it shouldn't be, and enter edit mode. In the properties panel, you should see the Object Data button, which appears as a triangle. In this tab you will see the Vertex Groups menu.



In this menu, select the leg bone in the list that affects the arm to move. Select all the armor mesh in the 3D viewport, then drag the Weight slider all the way down to 0.000. Then click assign. This takes the selected mesh, and sets how much it should move when the leg bone moves (which is now no movement).

Now when the leg bone is moved, the armor piece should not. Repeat this for all occurrences. This should also note that when moved to 1.000 it will move the mesh entirely (and anything in between will move accordingly with the weight value).